The aeration tank is used to decompose organic substances contained in dirty water, wastewater and sewage by cultivating aerobic microorganisms. The diffuser for aeration, which is fluid-communicated with a blower, is installed on the bottom of the aeration tank. The wastewater in the aeration tank is supplied with oxygen through air bubbles discharged from the diffuser for aeration. This oxygen is used to cultivate the aerobic microorganisms contained in the wastewater.
The filtration tank is used to eliminate solid particles contained in the wastewater through a filtration process. The filter tank is provided with a filtering unit such as, for instance, a submerged membrane filter module. The diffuser for aeration, which is fluid-communicated with the blower, is installed below the submerged membrane filter module in the filtration tank. The air bubbles discharged from the diffuser for aeration disturb water around the submerged membrane filter module. The membrane is subject to vibration due to “collision with the air bubbles” and “water disturbance around the membrane”, so that fouling phenomenon (micro-pores of the membrane are blocked by deposition of the solid particles) can be prevented.
In the bio-reactor, the solid particles are eliminated by the membrane module, and simultaneously, the organic substances are decomposed by the microorganisms. The air bubbles discharged from the diffuser for aeration installed on the bottom of the bio-reactor prevents the fouling of the membrane while supplying oxygen to the microorganisms.
The air bubbles discharged from the diffuser for aeration can be classified into micro air bubbles and macro air bubbles according to a diameter thereof. Typically, the air bubbles having a diameter from about 1 mm to about 3 mm are called micro air bubbles. The size of each discharged air bubble depends on a diameter of each air-bubble discharge hole. In general, the diameter of the air bubble is greater than the diameter of the air-bubble discharge hole. The micro air bubbles have an advantage in that they have high efficiency of transmitting the oxygen to the water. However, in order to increase physical cleaning effects of the air bubbles, it is advantageous to increase the size of the air bubbles discharged from the diffuser for aeration. For example, in the diffuser for aeration used for a membrane bio-reactor (MBR), the diameter of each air bubble discharge hole is typically designed within a range from about 1 mm to about 10 mm, and more frequently, within a range from about 3 mm to about 8 mm, by taking oxygen transmission effects and cleaning effects into consideration.
In the filter tank and the bio-reactor, the prevention of membrane fouling by the air bubbles discharged from the diffuser for aeration is called “physical cleaning of the membrane by the air bubbles”. With regard to the physical cleaning of the membrane by the air bubbles, the most fatal factor is the formation of a dead zone where a disturbance degree of the water is extremely low. In particular, in the case of the bio-reactor operating with high concentration microorganisms, if the dead zone is formed in the vicinity of the membrane, the surface blockade of the membrane may rapidly proceed due to sticky solid particles of microorganisms, and, pressure applied to the membrane may rapidly increase.
The dead zone is formed around the membrane mainly because the amount of air bubbles discharged from air bubble discharge holes of the diffuser for aeration is not uniform. This phenomenon is called “non-uniform aeration”. If the discharge amount of the air bubbles that serve as driving force for disturbing water is not uniform, the dead zone where a disturbance degree of the water is extremely low may be formed in the vicinity of the membrane.
FIG. 1 is a perspective view showing the conventional diffuser installation structure in which a membrane module fixing frame is integrally formed with a diffuser for aeration. The diffuser 100 for aeration, which is in the form of a rectangular pipe having a substantially U-shaped structure, is attached to a lower end portion of the frame 400. Air is fed into the diffuser 100 for aeration through an air feeding pipe 300. The membrane module is not shown in FIG. 1. The diffuser 100 for aeration is provided with a plurality of air bubble discharge holes which are directed upward. The frame 400 is installed in a filter tank while keeping the diffuser 100 for aeration in a horizontal state. If the amount of air bubbles discharged from some of air bubble discharge holes formed in the diffuser 100 for aeration is relatively small, the dead zone may be formed in the vicinity of the membrane module positioned above the corresponding air bubble discharge holes.
Hereinafter, the non-uniform aeration phenomenon occurred in the diffuser 100 for aeration will be described in detail with reference to FIG. 2. FIG. 2 is an enlarged sectional view showing a part of the diffuser 100 for aeration illustrated in FIG. 1. The diffuser 100 for aeration is provided with a plurality of air bubble discharge holes 111, 112 113, 114 and 115 which are directed upward. The airflow is indicated by arrows. The amount of air bubbles discharged from each air bubble discharge hole is indicated by the height of the vertical arrows. In the diffuser 100 for aeration, a region located above the dotted line is an air layer and a region located below the dotted line is a water layer. In general, since air-flow resistance increases proportionally to the distance relative to an air feeding port 101, the thickness of the air layer in the diffuser 100 for aeration becomes reduced proportionally to the distance relative to the air feeding port 101.
In the case of the submerged membrane system, typically, the diffuser for aeration capable of discharging macro air bubbles having a diameter of about 5 mm or above is preferred. However, as the diameter of the air bubble discharge holes 111, 112 113, 114 and 115 becomes enlarged, the pressure difference related to the airflow is reduced at each air bubble discharge hole, so that air is concentrated on the air bubble discharge holes (for instance, and 112), which are closest to the air feeding port 101. In an extreme case, air bubbles may not be discharged from the air bubble discharge hole (for instance, 115) located far away from the air feeding port 101. As a result, the dead zone may be formed above the air bubble discharge holes (for instance, 113 and 114) through which a relatively smaller amount of air bubbles are discharged, and especially, above the air bubble discharge hole 115 that does not discharge air bubbles.
The air bubble discharge holes (for instance, 113 and 114) through which a relatively smaller amount of air bubbles is discharged and the air bubble discharge hole 115 that does not discharge air bubbles are called “dead zone air bubble discharge holes”. One of important factors in operation of the water treatment facility is to reduce the number of the dead zone air bubble discharge holes.
In order to reduce the number of the dead zone air bubble discharge holes, there has been suggested a method of increasing the amount of air introduced into the air feeding port 101. If the amount of air fed into the air feeding port 101 is increased (although the amount of air bubbles discharged from the air bubble discharge hole closest to the air feeding port will be more increased), the air bubble discharge holes located far away from the air feeding port 101 may avoid being the “dead zone air bubble discharge holes”.
However, if the amount of air introduced into the air feeding port increases, the blower is subject to great load, the membrane is damaged due to the excessive discharge of air bubbles, and the operation cost for the water treatment facility is increased. In contrast, if it is possible to reduce the feed amount of air while reducing the number of the dead zone air bubble discharge holes, the operation cost can be saved.
Another method has been suggested to reduce the number of dead zone air bubble discharge holes. According to this method, the size of the air bubble discharge hole is adjusted while keeping the diffuser for aeration in a horizontal state. However, in this case, the non-uniform aeration phenomenon may not be sufficiently prevented. In addition, it is frequently necessary to set the diameter of the air bubble discharge holes, which are located adjacent to the air feeding port, to be smaller than the diameter required for physical cleaning.
In order to reduce the number of the dead zone air bubble discharge holes, there has been suggested another method of heightening the position of air bubble discharge holes fowled in the diffuser for aeration such that the air bubble discharge holes have a higher position proportionally to the distance relative to the air feeding port, while keeping the diffuser for aeration in a horizontal state. However, in this case, the thickness of a region filled only with water may increase at a lower portion of the distal end of the diffuser for aeration. The water remaining in the above region may stagnate because it does not receive shear force of air-flow. Thus, as time goes by, sludge is deposited, resulting in blockage of the air bubble discharge holes.
The further serious problem related to the diffuser for aeration is that most diffusers for aeration are designed under the condition that the diffusers for aeration are kept in the horizontal state. That is, under the precondition that the diffusers for aeration are kept in the horizontal state, the amount of air fed into the diffuser for aeration, the size of the air bubble discharge hole, and the position of the air bubble discharge hole are determined so that the non-uniform aeration phenomenon can be reduced.
However, in this case, when the diffuser for aeration is actually installed in the water treatment facility, if the diffuser for aeration is not precisely kept in the horizontal state, the diffuser for aeration does not achieve its intended design purpose, so that the non-uniform aeration may seriously occur. FIG. 3 is a view showing the diffuser for aeration which is not kept in the horizontal state. Since the diffuser 100 for aeration is not kept in the horizontal state, the distal end of the diffuser for aeration, which is located far away from the air feeding port 101, is sagged downward. Accordingly, the air layer, which is located above the dotted line, may not extend to the air bubble discharge holes 113, 114 and 115, and, the air bubble discharge holes 113, 114 and 115 are immersed in the water layer. As a result, the air bubble discharge holes 113, 114 and 115 cannot discharge air bubbles, thus, the air bubble discharge holes 113, 114 and 115 become dead zone bubble discharge holes.
Actually, in the diffuser system for aeration, which is integrally formed with a membrane module mounting frame, it is difficult to horizontally install the frame in the filter tank. In addition, even if the diffuser for aeration is provided separately from the frame, since it is difficult to horizontally maintain the diffuser for aeration on the bottom of the filter tank, the non-uniform aeration frequently occurs in the diffuser for aeration. Such a non-uniform aeration phenomenon can be relieved by increasing the amount of air fed into the diffuser for aeration, but this may cause the above-mentioned problems.
There are various attempts in the world to restrict the non-uniform aeration phenomenon of the diffuser of aeration. However, these attempts lead to the complex structure of the diffuser for aeration. Such a complex structure of the diffuser for aeration may increase the manufacturing cost of the diffuser for aeration and the installation cost of the water treatment facility.